Amino Acids, Proteins and Porphyrins
One nitrogen atom of urea is derived from
|B||Glycine of Aspartate|
|C||NH of Aspartate|
|D||Glutamate of Aspartate|
Fig: Reactions and intermediates of urea biosynthesis. The amines contributing to the formation of the
urea are shaded. Reaction 1 and 2 occur in the matrix of liver mitochondria and reactions 3, 4 and 5 in
CO2 (as bicarbonate), ammonium ion, and ornithine and citrulline traverse the mitochondrial matrix
via specific carriers (.) present in the inner membrane of liver mitochondria.
a. Synthesis of 1 mole of urea requires 3 mol of ATP and 1 mole each of ammonium ion (NH4+) and the α-amino nitrogen of Aspartate.
b. N-Acetyl glutamate functions solely as an enzyme activator. ie. Allosteric activator of carbamoyl phosphate synthase I.
c. In mammals, the major role of ornithine, citrulline, and arginino-succinate, is urea synthesis
d. Carbamoyl phosphate synthase-I is rate limiting or Pace-maker enzyme of the urea cycle. It is a hepatic mitochondrial enzyme.
e. Carbamoyl phosphate synthase-il, a cytosolic enzyme that uses glutamine rather than NH3 as the nitrogen donor, functions in pyrimidine biosynthesis.
f. Final step, is mediated by Arginase that release urea and L-ornithine from L-Arginine
g. L-glutamate is the only amino acid that under go oxidative deamination.